I would keep her well hydrated today and see if it doesn't lighten up. If it is the color of tea, very dark brown, that can sometimes mean liver problems. Pull down her eyelids. Are the whites of her eyes yellowish? If so, then see your vet. If not, then it is probably just dehydration. OH, and some medications cause darker urine as well. Push the fluids and then re-evaluate.
"Having heard all of this, you may choose to look the other way.....
but you can never say again that you did not know." William Wilberforce 1759-1833